(1) Field of the Invention
This invention relates to an optical multiplexer/demultiplexer comprising an arrayed waveguide grating and, more particularly, to an optical multiplexer/demultiplexer having a small-loss structure.
(2) Description of the Related Art
With an explosive increase in data traffic on networks, in recent years attention has been riveted to photonic networks on which a large amount of data can be transferred. To realize such networks, wavelength division multiplexing (WDM) optical communication networks are being built. An arrayed waveguide grating (AWG) in which the technology of a planar lightwave circuit (PLC) is adopted is a likely candidate for an optical wavelength multiplexer/demultiplexer essential to these WDM transmission systems.
FIG. 22 is a view showing the structure of a conventional arrayed waveguide grating.
As shown in FIG. 22, an arrayed waveguide grating 10 has the following waveguide structure. A sector slab waveguide 13 is connected to the output side of one or more optical input waveguides 12 arranged. An arrayed waveguide 14 is connected to the output side of the sector slab waveguide 13. A sector slab waveguide 15 is connected to the output side of the arrayed waveguide 14. A plurality of optical output waveguides 16 are connected to the output side of the sector slab waveguide 15. Usually the arrayed waveguide grating 10 is made by forming the above waveguide structure on, for example, a silicon substrate with cores made from siliceous glass or the like.
The sector slab waveguide 13 on the input side has the center of curvature at the end of the middle waveguide of the optical input waveguides 12. The sector slab waveguide 15 on the output side also has the center of curvature at the end of the middle waveguide of the optical output waveguides 16. The sector slab waveguides 13 and 15 have a structure in which the optic axes of waveguides in the arrayed waveguide 14 are located radially from the center of curvature. As a result, the optical arrangement of the sector slab waveguide 13 and arrayed waveguide 14 and of the sector slab waveguide 15 and arrayed waveguide 14 is the same as that of a concave mirror. That is to say, they will function the same as a lens. Moreover, in the arrayed waveguide 14, there is optical path length difference xcex94L between any two adjacent waveguides.
For example, the number of the optical input waveguides 12 and optical output waveguides 16 located corresponds to that of signal light beams with different wavelengths which are obtained as a result of demultiplexing by the arrayed waveguide grating 10 or which are to be multiplexed by the arrayed waveguide grating 10. Moreover, usually the arrayed waveguide 14 includes a large number of waveguides. In FIG. 22, for the sake of simplicity, only one optical input waveguide 12 is shown and the number of waveguides included in the arrayed waveguide 14 and optical output waveguide 16 is reduced.
If the arrayed waveguide grating 10 functions as an optical demultiplexer, light with a plurality of wavelengths xcex1, xcex2, . . . , xcexn is multiplexed by a WDM system and is input from the optical input waveguide 12 to the sector slab waveguide 13. This wavelength-multiplexed light spreads in the sector slab waveguide 13 by diffraction and is spreaded to each of the waveguides of the arrayed waveguide 14. In this case, the phases of light distributed to the waveguides of the arrayed waveguide 14 are the same. The light beams which propagated through the arrayed waveguide 14 are given phase difference corresponding to optical path length difference xcex94L between adjacent waveguides, interfere with one another in the sector slab waveguide 15 on the output side, and are condensed into the optical output waveguides 16. In this case, phase difference given in the arrayed waveguide 14 depends on the wavelengths, so the wavelengths are dispersed and the signal light beams are condensed into the different optical output waveguides 16 according to their wavelengths. As a result, the wavelength-multiplexed light input from the optical input waveguides 12 is demultiplexed into light with wavelengths of xcex1, xcex2, . . . , xcexn and is output from the different optical output waveguides 16.
Operation in the arrayed waveguide grating 10 is reversible. That is to say, if the direction in which light travels is inverted, the arrayed waveguide grating 10 will function as an optical multiplexer. Intervals xcex94xcex between the wavelengths of light obtained by demultiplexing are given approximately by:
xcex94xcex=(nsxc2x7dxc2x7nc)/(fxc2x7mxc2x7ng)xc2x7xcex94xxe2x80x83xe2x80x83(1)
where ns is an effective refractive index in the sector slab waveguides 13 and 15, d is a waveguide pitch at a portion where the arrayed waveguide 14 and sector slab waveguide 13 connect and at a portion where the arrayed waveguide 14 and sector slab waveguide 15 connect, nc is an effective refractive index in each of the waveguides of the arrayed waveguide 14, f is the focal length of the sector slab waveguides 13 and 15, m is a diffraction degree, ng is a group index in the arrayed waveguide 14, and xcex94x is an interval between adjacent optical output waveguides 16. If a center wavelength is xcex0, then m=(ncxc2x7xcex94L)/xcex0.
FIG. 23 is a graph showing an example of the passband characteristic of light demultiplexed in the above arrayed waveguide grating 10.
The passband characteristic of light obtained in each of the optical output waveguides 16 in the case of the arrayed waveguide grating 10 shown in FIG. 22 being used as an optical demultiplexer is shown in FIG. 23. In this case, the intensity of light obtained in each optical output waveguide 16 is highest at center wavelength xcex0 and becomes significantly lower at a wavelength farther from the center wavelength xcex0. In actual optical communication systems, however, moderately wide wavelength range R with the center wavelength xcex0 as its center will be used and there will be fluctuations in the wavelength of light propagating. As a result, with the above passband characteristic, the intensity of light obtained varies according to its wavelengths. In this case, shift D0 will occur. Therefore, a passband characteristic must be made flat so that the intensity of light obtained in the used wavelength range R will be constant.
FIG. 24 is a graph showing an example in which a passband characteristic is made flat.
On a graph shown in FIG. 24, a spectrum is flat in the used wavelength range R with the center wavelength xcex0 as its center. The intensity of light obtained is almost constant in this range and shift D1 in the intensity of the light is slight.
Conventionally, a Y branch circuit has been located at a portion where the optical input waveguide 12 and sector slab waveguide 13 connect in order to obtain light of constant intensity in the used wavelength range R. FIG. 25 is a view showing the structure of a Y branch circuit. FIG. 26 is a schematic view showing the shape of a mode of light output from the Y branch circuit to the sector slab waveguide 13. The x-axis in FIG. 26 is perpendicular to the waveguides of the optical input waveguide 12 or the arrayed waveguide 14.
As shown in FIG. 25, a Y branch circuit 17 has the shape of the letter xe2x80x9cYxe2x80x9d and is located at a portion where the optical input waveguide 12 and sector slab waveguide 13 connect. As a result, when single mode light which propagated through the optical input waveguide 12 is radiated into the sector slab waveguide 13 via the Y branch circuit 17, two peaks as shown in FIG. 26 will appear side by side in the shape of its mode. Therefore, two peaks also appear in the shape of a mode of light which is input from the sector slab waveguide 13 on the input side to the sector slab waveguide 15 on the output side through the arrayed waveguide 14 and which is condensed.
There is one peak at the center wavelength xcex0 in the shape of a mode of the optical output waveguides 16. In optical coupling of the shape of this mode and the shape of a mode of the sector slab waveguide 15 in which two peaks appear, a passband characteristic will be estimated approximately by an overlap integral of the two modes. Therefore, as shown by the graph in FIG. 24, in the optical output waveguides 16 light of constant intensity can be obtained in the used wavelength range R with the center wavelength xcex0 as its center.
With the above arrayed waveguide grating 10, however, excess loss will occur to output light by locating the Y branch circuit 17 at a portion where the optical input waveguide 12 and sector slab waveguide 13 connect. As shown in FIG. 25, this excess loss will increase with gap width W formed at a portion where the Y branch circuit 17 branches.
FIG. 27 is a graph showing the relationship between gap width W and excess loss in the Y branch circuit 17.
As shown in FIG. 27, excess loss caused by the Y branch circuit 17 increases in proportion to the gap width W. A gap several micrometers in width will be always formed in the Y branch circuit 17 for reasons of manufacture. Therefore, if the Y branch circuit 17 is used to make passband characteristics in the optical output waveguides 16 flat, it is impossible to reduce the amount of excess loss significantly.
In addition to this, with the arrayed waveguide grating 10 having the above structure, connection loss will occur between the sector slab waveguide 13 on the input side and the arrayed waveguide 14 regardless of whether a mode of input light is converted to make the passband characteristics of output light flat. The reason for the occurrence of this connection loss is as follows.
The shape of a mode of light input from the sector slab waveguide 13 to the arrayed waveguide 14 spreads significantly and horizontally. In contrast, the width of the shape of a mode of each waveguide of the arrayed waveguide 14 corresponds to that of a core, that is to say, the shape of a mode of each waveguide of the arrayed waveguide 14 is narrow. Therefore, the shape of a mode of the arrayed waveguide 14 obtained by synthesizing the shape of modes of all the waveguides of the arrayed waveguide 14 differs significantly from that of a mode of the sector slab waveguide 13. That is to say, the shape of a mode of the sector slab waveguide 13 does not match the shape of a mode of the arrayed waveguide 14. As a result, connection loss the amount of which corresponds to the difference between the shape of the two modes will occur.
The present invention was made under the background circumstances as described above. An object of the present invention is to provide an optical multiplexer/demultiplexer which can optimally make the passband characteristics of demultiplexed light flat by a small-loss structure.
Another object of the present invention is to provide an optical multiplexer/demultiplexer which can multiplex/demultiplex wavelength-multiplexed light by a small-loss structure.
In order to achieve the above objects, an optical multiplexer/demultiplexer having a waveguide structure comprising one or more optical input waveguides arranged, a first sector slab waveguide located on the output side of the optical input waveguides, an arrayed waveguide including a plurality of waveguides arranged any adjacent two of which differ in length by a constant value for propagating light output from the first sector slab waveguide, a second sector slab waveguide connected to the output side of the arrayed waveguide, and a plurality of optical output waveguides arranged and connected to the output side of the second sector slab waveguide is provided. In this optical multiplexer/demultiplexer, the optical input waveguides and the first sector slab waveguide are connected by a directional coupler having a symmetrical structure comprising a central waveguide including an end portion on the first sector slab waveguide side of the optical input waveguides on both side portions of which a taper is formed so that the width of a core will gradually narrow in the direction of the end portion, and not touching the first sector slab waveguide and a plurality of arranged waveguides one end of each of which is connected to the input side of the first sector slab waveguide and which are arranged on both sides of the central waveguide by the same numbers.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.